All birds Blog: Bean Geese (3) - more from Shetland

Hugh Harrop sent me some new pictures from the Bean geese from Shetland. He noted: ‘ They are clearly very interesting birds and I continue to scratch my head! The bills are clearly interesting!’

Yes, they are, I immediately thought. In previous decades, identifying Beans was all about bills. Yellowbilled, huge Bean geese were fabalis, and short-blackbilled were rossicus. It is logic to focus on their bills, because there are no obvious other characteristics of Beans that differ in such way. Say ‘character’ and’ Bean’, you have to use the word ‘slightly’, because every difference between rossicus and fabalis is slight: bill, size and structure.
But now only about the bills. According to Roselaar (1977), one of the few morphological studies about the Bean geese complex, the structure of the bill is important, but difficult to measure.

Taiga Beans (fabalis) do have a long and thin upper mandible, which sides are parallel, the nail is round (looking right from above), lower mandible is straight and thin.
Tundra Beans (rossicus) do have a shorter and higherbased upper mandible. The sides are converging towards the tip. The nail is more oval and the lower mandible thicker and strongly curved.
That bill structure is more important than bill length, shows table 1. Bill length does only differ some millimeters between rossicus and fabalis. On average, the bill of a fabalis is slightly longer. The only significant difference in billmeasurements Ruokonen et al (2008) found, was the height of the grinning patch: rossicus 7.29 (n=10) and fabalis 5.8 (n=17). Also slightly!
What are we talking about?! I hope clear it up a bit with these pictures:

1a Rossicus - Thick lower mandibles like these are quite common in flocks in the Netherlands. Interestingly, they show up more in the late winter. Are they from more eastern parts of the Rossicus area? 20/1/2011 in NL - Max Berlijn

1b. Rossicus. Bills like these are the most common: a curved lower mandible, less orange than black. - Albert de Jong


1c. Rossicus - With thick lower mandible - 2/1/2012 - Albert de Jong


2. Rossicus - Male, shot in 1854 (specimen) in NL - Fred Visscher


3. Fabalis - female, 25/2/1871 in NL - Fred Visscher


4. Fabalis - Note the thin lower mandibles. The bill length of the bird on the right isn't very long. February 2011 in NL - Albert de Jong Have a look at these birds now. What about their bills?

Shetland 1

Shetland 2

Shetland 3

Shetland 4

Or what about this stranger on 8th January 2011 in The Wieringermeer, The Netherlands ? We discussed this bird on Dutch Birding. The positive ID as a female fabalis was confirmed by expert Thomas Heinicke as you can read. Looking only at the bill, we can see the lower mandible isn't curved downwards, but straight and thin. Towards the tip, the upper mandible overlaps the lower a bit, which is seen regularly in fabalis. See also 'Shetland 1' above, that shows two slender-billed birds like the bird here below. Shetland 3 also shows such a bill. Shetland 2 has a shorter bill than you possibly expect in fabalis and more in rossicus, but as table 1 shows, length doesn't matter that much. Amongst the biggest Dutch flock of up to 70 birds of last winter, described here, there were some (females I think) with bills like this. The structure of this bill is typical for fabalis. So far, the focus on one aspect of the whole picture you need to ID Beans. Hugh also noted that the size of the Shetland birds is 'to be slightly smaller than the (small) Icelandic Greylags we have here and not much bigger than the 'biggest' Tundra Beans in the same flock.' I think that is a good description for fabalis, since the more delicate structure and size of fabalis is somewhere between the more bulcky Greylags and Tundra´s. Structure does matter may be more than size! But is also more difficult to describe. The future job is to get the right picture(s) of the variability in fabalis and rossicus. I think there has been a lot of improvement in the last years.....